1. Field of the Invention
The present invention relates to a DC-offset transient response cancel system which cancels out a transient response of a DC offset which deteriorates the voice quality in a direct conversion system, i.e., a system which will be the mainstream system in the future for mobile telephones. A transient response of a DC offset worsens a bit error rate and makes it impossible to correctly decode a transmitted signal.
2. Background Art
Over the recent years, more and more mobile telephone systems are shifting from heterodoxy conversion systems to direct conversion systems which operate at low power and are advantageous to size reduction. However, a direct conversion system has a problem of a DC offset from which a heterodoxy conversion system is free.
A prior art on direct conversion systems will now be described.
FIG. 4 is a block diagram which shows the prior art on a direct conversion system. In FIG. 4, denoted at 1 is a low noise amplifier (linear amplifier). Denoted at 2 is a first capacitor for capacitance-coupling. Denoted at 3 is a demodulator which down-converts a high frequency signal (RF signal input) which is a received signal. Denoted at 4 is a first gain control amplifier (hereinafter referred to as a “GCA”). Denoted at 5 is a second capacitor for capacitance-coupling (having the capacitance of 200 pF for instance). Denoted at 6 is a first resistor (having the resistance value of 2000 kΩ for instance) which forms a high pass filter together with the second capacitor 5. Denoted at 7 is a second GCA. Denoted at 8 is a third capacitor for capacitance-coupling (having the capacitance of 200 pF for instance). Denoted at 9 is a second resistor (having the resistance value of 2000 kΩ for instance) which forms a high pass filter together with the third capacitor. Denoted at 10 is a third GCA. Denoted at 11 is a GCA control circuit which controls the gains of the GCA 4, the GCA 7 and the GCA 10 in accordance with a gain control signal GCV. Denoted at 24 is a first bias power source which applies through the first resistor 6 a bias voltage upon a signal after capacitance-coupling. Denoted at 25 is a second bias power source which applies through the second resistor 9 a bias voltage upon a signal after capacitance-coupling.
Operations of the direct conversion system having such a structure above will now be described. First, the RF signal amplified by the low noise amplifier 1 is capacitance-coupled by the first capacitor 2 and then fed to the demodulator 3. The demodulator 3 down-converts the RF signal, and the RF signal is accordingly frequency-converted to baseband and becomes an I/Q−BB (in phase/quadrant phase−baseband) signal. Thus frequency-converted signal is amplified by the GCA 4 to a desired level, capacitance-coupled by the second capacitor 5 and the first resistor 6, and then fed to the GCA 7 in a condition that the signal has a DC value set by the bias power source 24.
The signal is further amplified by the GCA 7 to a desired level, capacitance-coupled by the third capacitor 8 and the second resistor 9, and then fed to the GCA 10 in a condition that the signal has a DC value set by the bias power source 25.
The signal is amplified by the GCA 10 again to a desired level, and outputted as an RX−I/Q output (receipt−in phase/quadrature phase signal output).
The gains of the GCA 4, the GCA 7 and the GCA 10 are controlled via the GCA control circuit 11, referring to the gain control signal GCV which changes in accordance with the level of the RF signal.
FIG. 6A shows one example of the inside of the GCA control circuit 11. In FIG. 6A, denoted at 26 is a gain control signal generator for the GCA 4. Denoted at 27 is a gain control signal generator for the GCA 7. Denoted at 28 is a gain control signal generator for the GCA 10. The gain control signal generators 26, 27 and 28 generate gain control signals respectively for the GCA 4, the GCA 7 and the GCA 10 in accordance with the gain control signal GCV. The signal timing is as shown in the timing chart in FIG. 6B.
The GCA control circuit 11 maybe an analog control circuit which controls while referring to a voltage for instance, or a logic control circuit which controls serially for example.
However, the prior art above gives rise to a problem when a DC offset is created in the circuit because of a variation among elements. In short, owing to the capacitance-coupling, static operations lead to no problem. Despite this, when the input level of the RF signal abruptly changes, that is, when the gains of the GCA 4, the GCA 7 and the GCA 10 suddenly change as the gain control signal GCV suddenly changes, a DC-offset transient response is created. The waveforms at the respective portions will now be described with reference to FIG. 5A. FIG. 5A shows a base band part alone. For simplicity of illustration, AC components are omitted from the waveforms which are shown in FIG. 5A.
An example that the gains of the GCA 4, the GCA 7 and the GCA 10 grow at some moment will now be described on the assumption that the GCA 4, the GCA 7 and the GCA 10 have positive input offsets which are attributed to variations. First, as the gain of the GCA 4 changes, the DC level rises at that moment, and therefore, the output from the GCA 4 becomes as denoted at a waveform 12. As the DC value having the waveform 12 passes the capacitor 5, the DC value becomes as denoted at a waveform 13. Since a baseband signal exists even at frequencies close to DC, for the purpose of lowering the cut-off frequency of the high pass filter, the time constants of the capacitor 5 and the resistor 6 must be large. Because of this, the waveform 13 needs time until the DC level reaches a steady value. In other words, after momentarily growing following the DC value having the waveform 12, the DC level gradually returns to the original DC value. In the GCA 7 as well, the DC value becomes as denoted at a waveform 14 since the DC level grows in response to a change in gain. As the DC value having the waveform 14 passes the capacitor 8, the DC value becomes as denoted at a waveform 15 through similar operations to those described above. Also in the GCA 10, the DC value becomes as denoted at a waveform 16 since the DC level grows in response to a change in gain. On this occasion, the time the DC value takes before settling at a steady value after changing is a transient response. A DC-offset transient response arises in this manner. The timing of each waveform is as shown in the timing chart in FIG. 5B.